An adhesive layer is often required and desired to attach or bond two or more structures together in order to form an assembly that performs a function. As an example, a rotor blade of a rotorcraft is manufactured through several different adhesive bonding operations of many individual components to form a final blade assembly that will help give the aircraft lift while maintaining its structural integrity throughout all flight conditions and environments. Since the individual components of an assembly are shaped differently, made of different materials, and/or are affected differently by the various operational conditions, they wear and degrade at different rates. Also, a layer of primer, or the like, may be applied to the underlying structure and/or the overlaying structure may be applied to help ensure adhesion. Additionally, or alternatively, composite materials, which comprise at least one structural material and a matrix material, are used extensively in modern aircraft.
Aircraft are often required to fly in extreme environments such as sandy desert, rain or thunderstorms, around saltwater and/or in combat zones. In the case of rotorcraft, exposure to the elements can cause significant damage to rotor blade components over time, which often leads to repairs or replacement. Common damage that results in subcomponent removal of rotor blades includes erosion to the leading edge metal abrasion strip, ice protection failures to the heater blanket disposed beneath the metallic abrasion strip, impact damage to the upper and lower skins that cover the afterbody surfaces of the blade, and the like.
It is common to remove, repair, and replace certain damaged or degraded components of an assembly while reusing others. However, the methods of disassembly used today can be unsafe, labor-intensive, costly, disordered and damaging to the underlying or adjacent structure. In the case where removal, repair, and replacement of the damaged or degraded component is too difficult or costly, beyond economic repair, the components are often scrapped and replaced.
Traditional methods for disassembly of bonded components have involved the use of heat or other temperature degradation or embrittlement, electrical degradation, or mechanical disassembly of such components using force and/or sharp tools, all of which often cause damage to components intended to be salvaged. In the example of the removal of a damaged rotor blade component on a rotorcraft, the process is typically an operator-dependent and time-consuming process. Such removal of a damaged rotor blade component often employs a method that requires a high degree of craftsmanship and may be physically exhaustive. Various methods for removal of a damaged rotor blade component have been developed over the years. Such methods have been characterized as “messy,” requiring specified tooling and support equipment, unsafe, risky in terms of both operator safety and/or potentially incurred damage to the component or assembly. Further, such removal of damaged rotor blade components may be limited to facilities with certain resources and infrastructure. With these existing methods of rotor blade component disassembly there is often unintentional damage incurred to the remaining structure of the blade. This may lead to increased cost for rework, and in many cases, scrapping of the component(s). As a typical, specific example, it is almost impossible to remove a good metallic abrasion strip without damaging it in order to replace a failed heater blanket that is disposed beneath the abrasion strip, at least in a reasonable amount of time. Conversely, in the case of an eroded abrasion strip, the removal process typically damages the underlying heater blanket, which may be functional and not in need of replacement.
These issues with the difficulty of disassembly of such components may be considered to result from design considerations for such a blade, expecting an abrasion strip and heater blanket to stay on the blade indefinitely. Just like all bonded structures, producibility and performance concerns revolve around strong and durable adhesion. Hence, as discussed above, good rotor blade components are often sacrificed during the removal process of a single damaged or failed part.
A specific example of removal of a failed heater blanket or eroded metal abrasion strip, using heat gun, wedges, hammers, chisels, and the like, includes an operator using the heat gun to soften the underlying bondline in order to slip in wedges under the edges of the metallic substrate all the way around the abrasion strip. Then, a large hammer or mallet is used to apply force to the wedges in order to locally dis-bond the abrasion strip. Pliers or mandrels may also be used to help peel back the metal of the abrasion strip, such as in long narrow bands (i.e. in can key strip opening fashion). Once the abrasion strip is removed, the thin heater blanket layer is chiseled away. Then, the remaining heater blanket material and adhesive remaining on underlying components is sanded down, in attempt to reveal an undamaged rotor spar or sheath underneath the heater blanket. Often, the hammering of wedges or chiseling leads to damage to spar or sheath plies, resulting in a scrapped part or rotor blade, particularly if a local repair to the spar or sheath plies is not sufficient. This removal of a failed heater blanket or eroded metal abrasion strip is a time-consuming, unclean, and costly method for rotor blade component removal that does not always result in success.
In another specific example of removal of a failed heater blanket or eroded abrasion strip using dry ice, wedges, hammers, and the like, there is more potential for successful abrasion strip removal, while preserving the underlying heater blanket. Therein, a dry ice bath is used to lower the temperature of the metal substrate of the abrasion strip. This method requires more, typically a considerable amount of more, special tooling and equipment that is tailored to the specific blade in order to acquire consistent cooling across the abrasion strip to be removed. The dry ice can be dangerous to personnel and must be handled with care. This is also a very tedious and operator dependent process when done correctly. The temperature of the metal must be tightly controlled for successful removal, so the time in the dry ice bath must be closely monitored. The part, or overall blade structure, can be permanently damaged if left in the dry ice bath too long. The cooling of the metal causes thermal contraction/expansion such that the part (metal adhesion strip) moves relative to the underlying blade structure. The idea is to create a clean disbond with this process, without damaging any components. When a clean disbond is not accomplished, sometimes hammers and wedges are used at the part edges. There is still considerable room for error in this process and it may not save any time or cost over other methods, such as use of a heat gun, as described above, use of an electrical current to degrade the bond, or the like.